Graphene and power storage device, and manufacturing method thereof

ABSTRACT

The formation method of graphene includes the steps of forming a layer including graphene oxide over a first conductive layer; and supplying a potential at which the reduction reaction of the graphene oxide occurs to the first conductive layer in an electrolyte where the first conductive layer as a working electrode and a second conductive layer with a as a counter electrode are immersed. A manufacturing method of a power storage device including at least a positive electrode, a negative electrode, an electrolyte, and a separator includes a step of forming graphene for an active material layer of one of or both the positive electrode and the negative electrode by the formation method.

TECHNICAL FIELD

The present invention relates to formation methods of graphene and anelectrode including the graphene, and a manufacturing method of a powerstorage device including the electrode. The present invention alsorelates to graphene and an electrode which are formed by the formationmethods and a power storage device which is manufactured by themanufacturing method. Note that a power storage device in thisspecification refers to every element and/or device having a function ofstoring electric power, such as a lithium primary battery, a lithiumsecondary battery, or a lithium-ion capacitor.

BACKGROUND ART

In recent years, attempts have been made to apply graphene to a varietyof products because of its excellent electric characteristic of highconductivity and its excellent physical characteristics such assufficient flexibility and high mechanical strength.

Application of graphene to power storage devices such as a lithiumsecondary battery and a lithium-ion capacitor is one of the attempts.For example, an electrode material can be coated with graphene toimprove the conductivity of the electrode material for a lithiumsecondary battery.

As a method for forming graphene, a method of reducing graphite oxide orgraphene oxide in the presence of a base is given. In order to formgraphite oxide using the method for forming graphene, a method usingsulfuric acid, nitric acid, and potassium chlorate as an oxidizer, amethod using sulfuric acid and potassium permanganate as an oxidizer, amethod using potassium chlorate and fuming nitric acid as an oxidizer,or the like can be employed (see Patent Document 1).

As a method of forming graphite oxide with the use of sulfuric acid andpotassium permanganate as an oxidizer, the modified Hummers method isgiven. Here, the method of forming graphene by the modified Hummersmethod will be described with reference to FIG. 14.

Graphite is oxidized using an oxidizer such as potassium permanganate ina solvent; thus, a mixed solution 1 containing graphite oxide is formed.After that, in order to remove the remaining oxidizer in the mixedsolution 1, hydrogen peroxide and water are added to the mixed solution1, and a mixed solution 2 is formed (Step S101). Here, unreactedpotassium permanganate is reduced by the hydrogen peroxide and then thereduced potassium permanganate reacts with sulfuric acid, wherebymanganese sulfate is formed. Then, the graphite oxide is collected fromthe mixed solution 2 (Step S102). Then, the collected graphite oxide iswashed with an acid solution in order to remove the oxidizer whichremains in or is attached to the graphite oxide, and subsequently, thegraphite oxide is washed with water (Step S103). Note that the washingstep Step S103 is performed repeatedly. After that, the graphite oxideis diluted with a large amount of water and centrifuged, and thegraphite oxide from which an acid is separated is collected (Step S104).Then, ultrasonic waves are applied to a mixed solution containing thecollected graphite oxide and an oxidized carbon layer in the graphiteoxide is separated, so that graphene oxide is formed (Step S105). Then,the graphene oxide is reduced, whereby graphene can be formed (StepS106).

For a method of forming graphene by reducing graphene oxide, heattreatment can be employed.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2011-500488

DISCLOSURE OF INVENTION

In some cases, the conductivity of graphene formed by reducing grapheneoxide depends on the bonding state in the graphene.

In view of the above, an object of one embodiment of the presentinvention is to provide graphene which is formed from graphene oxide andhas high conductivity and to provide a method for forming the graphene.

An electrode included in a power storage device includes a currentcollector and an active material layer. In a conventional electrode, anactive material layer includes a conductive additive, binder, and/or thelike as well as an active material. For this reason, it is difficult toefficiently increase only the weight of the active material in anelectrode, and thus, it is difficult to increase the charge anddischarge capacity per unit weight or volume of the electrode. Further,the conventional electrode also has a problem in that the binderincluded in the active material layer swells as it comes into contactwith an electrolyte, so that the electrode is likely to be deformed andbroken.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a power storage device with high chargeand discharge capacity per unit weight or volume of an electrode, highreliability, high durability, and the like and to provide a method formanufacturing the power storage device.

Oxides such as graphite oxide and graphene oxide can be reduced throughheat treatment. In the present invention, however, graphene oxide iselectrochemically reduced with electric energy to form graphene. In thisspecification, reduction caused by supplying a potential for promotingthe reduction reaction of an active material layer may be referred to aselectrochemical reduction.

In this specification, graphene refers to a one-atom-thick sheet ofcarbon molecules with a gap through which ions can pass and double bonds(also referred to as sp² bonds), or a stack of 2 to 100 layers of thesheets. The stack can also be referred to as multilayer graphene.Further, in the graphene, the proportion of an element other thanhydrogen and carbon is preferably 15 at. % or lower, or the proportionof an element other than carbon is preferably 30 at. % or lower. Notethat graphene to which an alkali metal such as potassium is added may beused. Thus, an analog of graphene is included in the category of thegraphene.

Further, graphene oxide in this specification refers to graphene inwhich an oxygen atom is bonded to a six-membered ring or a many-memberedring each composed of carbon atoms. Specifically, graphene oxide in thisspecification refers to graphene in which an epoxy group, a carbonylgroup such as a carboxyl group, a hydroxyl group, or the like is bondedto a six-membered ring or a many-membered ring each composed of carbonatoms. In graphene oxide, graphene oxide salt is formed in some casesdepending on a formation method. The graphene oxide salt refers to, forexample, a salt in which ammonia, amine, an alkali metal, or the likereacts with an epoxy group, a carbonyl group such as a carboxyl group,or a hydroxyl group bonded to a six-membered ring or a many-memberedring each composed of carbon atoms. In this specification, “grapheneoxide” includes “graphene oxide salt” in its category. Note thatgraphene oxide and graphene oxide salt each include one sheet or a stackof 2 to 100 layers of the sheets, and the stack can also be referred toas multilayer graphene oxide or multilayer graphene oxide salt.

One embodiment of the present invention is a method for forminggraphene. The method includes the steps of forming a layer includinggraphene oxide over a first conductive layer; and supplying a potentialat which the reduction reaction of the graphene oxide occurs to thefirst conductive layer in an electrolyte in which the first conductivelayer as a working electrode and a second conductive layer as a counterelectrode are immersed, so that graphene is formed. Specifically, thepotential supplied to the first conductive layer is set to 1.6 V to 2.4V inclusive (the redox potential of lithium is used as a referencepotential), a potential at which the reduction reaction of the grapheneoxide occurs, and the graphene oxide is reduced to form graphene. Notethat the case where the redox potential of lithium is used as areference potential may be hereinafter denoted as “vs. Li/Li⁺”.

One embodiment of the present invention is a method for forminggraphene. The method includes the steps of forming a layer includinggraphene oxide over a first conductive layer; and sweeping the potentialof the first conductive layer so that it includes at least a potentialat which the reduction reaction of the graphene oxide occurs in anelectrolyte in which the first conductive layer as a working electrodeand a second conductive layer as a counter electrode are immersed andreducing graphene oxide, so that graphene is formed. Specifically, asdescribed above, the potential of the first conductive layer is swept soas to include the range of 1.4 V to 2.6 V (vs. Li/Li⁺), a potential atwhich the graphene oxide can be reduced, preferably the range of 1.6 Vto 2.4 V (vs. Li/Li⁺). Further, the potential of the first conductivelayer may be periodically swept so as to include the range. Periodicalpotential sweeping enables sufficient reduction of the graphene oxide.

A power storage device can be manufactured using any of the abovemethods. One embodiment of the present invention is a method formanufacturing a power storage device including at least a positiveelectrode, a negative electrode, an electrolyte, and a separator. Themethod includes the steps of forming an active material layer includingat least an active material and graphene oxide, over a currentcollector, in one of or both the positive electrode and the negativeelectrode; and supplying a potential at which the reduction reaction ofthe graphene oxide occurs to the current collector, so that graphene isformed. Specifically, the potential supplied to the current collector inone of or both the positive electrode and the negative electrode is setto 1.4 V to 2.6 V inclusive (vs. Li/Li⁺), preferably 1.6 V to 2.4 Vinclusive (vs. Li/Li⁺), and the graphene oxide is reduced to formgraphene.

One embodiment of the present invention is a method for manufacturing anelectrode and a power storage device including the electrode. The methodfor manufacturing the electrode includes the steps of forming an activematerial layer including at least an active material and graphene oxideover a current collector; and sweeping the potential of the currentcollector so that it includes at least a potential at which thereduction reaction of the graphene oxide occurs and reducing thegraphene oxide, so that graphene is formed. Specifically, as describedabove, the potential of the current collector is swept so as to includethe range of 1.4 V to 2.6 V (vs. Li/Li⁺), a potential at which thegraphene oxide can be reduced, preferably the range of 1.6 V to 2.4 V(vs. Li/Li⁺). At this time, graphene is formed on a surface of theactive material or in the active material layer. The potential of thecurrent collector may be periodically swept so as to include the range.Periodical sweeping of the potential of the current collector enablessufficient reduction of the graphene oxide in the active material layer,for example.

In graphene formed by the above method for forming graphene, theproportions of carbon atoms and oxygen atoms, which are measured byX-ray photoelectron spectroscopy (XPS), are 80% to 90% inclusive and 10%to 20% inclusive, respectively. Further, in the graphene, the proportionof sp²-bonded carbon atoms of the carbon atoms measured by XPS is 50% to80% inclusive, preferably 60% to 70% inclusive or 70% to 80% inclusive,i.e., 60% to 80% inclusive.

Note that one embodiment of the present invention includes a powerstorage device which includes the graphene in one of or both a positiveelectrode and a negative electrode.

According to one embodiment of the present invention, graphene which hasa higher proportion of C(sp²)-C(sp²) double bonds and higherconductivity than graphene formed through heat treatment and amanufacturing method of the graphene can be provided. Moreover, a powerstorage device whose charge and discharge capacity per unit weight,reliability, and durability are high and a manufacturing method of thepower storage device can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates a formation method of graphene of one embodiment ofthe present invention, and FIG. 1B illustrates an apparatus used forforming the graphene;

FIG. 2 illustrates a formation method of graphene oxide of oneembodiment of the present invention;

FIG. 3 illustrates a formation method of graphene oxide of oneembodiment of the present invention;

FIGS. 4A to 4C illustrate a positive electrode of one embodiment of thepresent invention;

FIGS. 5A to 5D illustrate a negative electrode of one embodiment of thepresent invention;

FIG. 6 illustrates a power storage device of one embodiment of thepresent invention;

FIG. 7 illustrates electric appliances;

FIGS. 8A to 8C illustrate an electric appliance;

FIG. 9 shows a result of cyclic voltammetry measurement;

FIG. 10 shows a result of cyclic voltammetry measurement;

FIG. 11 shows a result of cyclic voltammetry measurement;

FIG. 12 shows XPS analyses of the composition of surface elements;

FIG. 13 shows XPS analyses of the states of atomic bonds;

FIG. 14 illustrates a conventional formation method of graphene;

FIGS. 15A and 15B each show a result of cyclic voltammetry measurement;and

FIGS. 16A and 16B each show a result of cyclic voltammetry measurement.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and examples of the present invention will be describedbelow with reference to the drawings. Note that the present invention isnot limited to the following description, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Thus, the present invention should not beinterpreted as being limited to the following description of theembodiments and examples. In description using the drawings forreference, in some cases, common reference numerals are used for thesame portions in different drawings. Further, in some cases, the samehatching patterns are applied to similar portions, and the similarportions are not necessarily designated by reference numerals.

Embodiment 1

In this embodiment, a method for forming graphene of one embodiment ofthe present invention will be described below with reference to FIGS. 1Aand 1B. FIG. 1A is a flow chart showing a process of forming graphene,and FIG. 1B is a schematic view of an apparatus used to form graphene.

According to the method for forming graphene of one embodiment of thepresent invention, to form graphene, graphene oxide is not reducedthrough heat treatment but electrochemically reduced with electricenergy.

<Step S111>

In Step S111 shown in FIG. 1A, a layer including graphene oxide isformed on a surface of a conductive layer. For example, a dispersionliquid containing graphene oxide is applied to the conductive layer. Asthe dispersion liquid containing graphene oxide, a commercial product ora dispersion liquid obtained by dispersing graphene oxide formed by themethod described with reference to FIG. 14, or the like, in a solventmay be used. Alternatively, a dispersion liquid obtained by dispersinggraphene oxide (graphene oxide salt) formed by the following method in asolvent may be used.

The conductive layer can be formed using any material as long as thematerial has conductivity. For example, a metal material such asaluminum (Al), copper (Cu), nickel (Ni), or titanium (Ti) or an alloymaterial containing some of the above metal materials can be used. Asthe alloy material, for example, an Al—Ni alloy and an Al—Cu alloy canbe given. The conductive layer can have a foil shape, a plate shape, anet shape, or the like as appropriate, and the metal material or thealloy material which is formed over a substrate and separated may beused as the conductive layer.

As a method of applying the dispersion liquid containing graphene oxideto the conductive layer, a coating method, a spin coating method, a dipcoating method, a spray coating method, and the like can be given.Alternatively, these methods may be combined as appropriate. Forexample, after the dispersion liquid containing graphene oxide isapplied to the conductive layer by a dip coating method, the conductivelayer is rotated as in a spin coating method, so that the evenness ofthe thickness of the applied dispersion liquid containing graphene oxidecan be improved.

After the dispersion liquid containing graphene oxide is applied to theconductive layer, the solvent in the dispersion liquid is removed. Forexample, drying is performed in vacuum for a certain period of time toremove the solvent from the dispersion liquid containing graphene oxidewhich is applied to the conductive layer. Note that time needed forvacuum drying depends on the amount of applied dispersion liquid. Thevacuum drying may be performed while heating is performed as long as thegraphene oxide is not reduced. For example, to make the thickness of thegraphene oxide after Step S111 approximately 10 μm, it is preferable toperform vacuum drying for approximately one hour while the conductivelayer is heated at a temperature higher than or equal to roomtemperature and lower than or equal to 100° C. and to perform vacuumdrying at room temperature for approximately one hour.

<Step S112>

Next, the graphene oxide formed on the conductive layer is reduced toform graphene. In this step, the graphene oxide is electrochemicallyreduced using electric energy as describe above. When this step isschematically described, in this step, a closed circuit is formed withthe use of the conductive layer provided with the graphene oxide, whichis obtained in Step S111, and a potential at which the reductionreaction of the graphene oxide occurs or a potential at which thegraphene oxide is reduced is supplied to the conductive layer, so thatthe graphene oxide is reduced to form graphene. Note that in thisspecification, a potential at which the reduction reaction of thegraphene oxide occurs or a potential at which the graphene oxide isreduced is referred to as the reduction potential.

A method for reducing the graphene oxide will be specifically describedwith reference to FIG. 1B. A container 113 is filled with an electrolyte114, and a conductive layer 115 provided with the graphene oxide and acounter electrode 116 are put in the container 113 so as to be immersedin the electrolyte 114. In this step, an electrochemical cell (opencircuit) is formed with the use of at least the counter electrode 116and the electrolyte 114 besides the conductive layer 115 provided withthe graphene oxide, which is obtained in Step S111, as a workingelectrode, and the reduction potential of the graphene oxide is suppliedto the conductive layer 115 (working electrode), so that the grapheneoxide is reduced to form graphene. Note that an aprotic organic solventsuch as ethylene carbonate or diethyl carbonate can be used as theelectrolyte 114. Note that the reduction potential to be supplied is areduction potential in the case where the potential of the counterelectrode 116 is used as a reference potential or a reduction potentialin the case where a reference electrode is provided in theelectrochemical cell and the potential of the reference electrode isused as a reference potential. For example, when the counter electrode116 and the reference electrode are each made of lithium metal, thereduction potential to be supplied is a reduction potential determinedrelative to the redox potential of the lithium metal (vs. Li/Li⁺).Through this step, reduction current flows through the electrochemicalcell (closed circuit) when the graphene oxide is reduced. Thus, toexamine whether the graphene oxide is reduced, the reduction currentneeds to be checked sequentially; the state where the reduction currentis below a certain value (where there is no peak corresponding to thereduction current) is regarded as the state where the graphene oxide isreduced (where the reduction reaction is completed).

In controlling the potential of the conductive layer 115 in this step,the potential of the conductive layer 115 may be fixed to the reductionpotential of the graphene oxide or may be swept so as to include thereduction potential of the graphene oxide. Further, the sweeping may beperiodically repeated like in cyclic voltammetry. Although there is nolimitation on the sweep rate of the potential of the conductive layer115, it is preferably 0.005 mV/s to 1 mV/s inclusive. Note that thepotential of the conductive layer 115 may be swept either from a higherpotential to a lower potential or from a lower potential to a higherpotential.

Although the reduction potential of the graphene oxide slightly variesdepending on the structure of the graphene oxide (e.g., the presence orabsence of a functional group and formation of graphene oxide salt) andthe way to control the potential (e.g., the sweep rate), it isapproximately 2.0 V (vs. Li/Li⁺). Specifically, the potential of theconductive layer 115 may be controlled so as to fall within the range of1.4 V to 2.6 V (vs. Li/Li⁺), preferably the range of 1.6 V to 2.4 V (vs.Li/Li⁺). The details of the reduction potential of the graphene oxidewill be described in examples below.

Through the above steps, the graphene can be formed on the conductivelayer 115.

In the graphene formed by the method for forming graphene of oneembodiment of the present invention, the proportions of carbon atoms andoxygen atoms, which are measured by XPS, are 80% to 90% inclusive and10% to 20% inclusive, respectively. The proportion of sp²-bonded carbonatoms of the carbon atoms is 50% to 80% inclusive, preferably 60% to 70%inclusive or 70% to 80% inclusive, i.e., 60% to 80% inclusive.

As a method for reducing graphene oxide, other than a method ofelectrochemical reduction with electric energy, a method of causingreduction by releasing oxygen atoms in graphene oxide as carbon dioxidethrough heat treatment (also referred to as thermal reduction). Thegraphene of one embodiment of the present invention is different fromgraphene formed by thermal reduction in at least the following points.Since the graphene of one embodiment of the present invention is formedby electrochemically reducing the graphene oxide with electric energy,the proportion of C(sp²)-C(sp²) double bonds is higher than that ingraphene formed by thermal reduction. Thus, the graphene of oneembodiment of the present invention has more π electrons which are notlocalized in a particular position and are broadly conducive tocarbon-carbon bonds than graphene formed by thermal reduction, whichsuggests that the graphene of one embodiment of the present inventionhas higher conductivity than graphene formed by thermal reduction.

In the method described with reference to FIG. 14 as an example of amethod for forming graphene oxide which can be employed in Step S111, alarge amount of water is necessary in Step S103, the step of washinggraphene oxide. When Step S103 is repeated, acid can be removed fromgraphite oxide. However, when the acid content thereof becomes low, itis difficult to separate the graphite oxide, which is a precipitate, andacid contained in a supernatant fluid; accordingly, the yield of thegraphite oxide may probably be low, leading to a lower yield ofgraphene.

Here, a method for forming graphene oxide which is different from themethod described with reference to FIG. 14 in Step S111 will bedescribed.

FIG. 2 is a flow chart showing a process of forming graphene oxide (orgraphene oxide salt).

<Oxidation Treatment of Graphite>

As shown in Step S121, graphite is oxidized with an oxidizer to formgraphite oxide.

As an oxidizer, sulfuric acid, nitric acid and potassium chlorate;sulfuric acid and potassium permanganate; or potassium chlorate andfuming nitric acid are used. Here, graphite is oxidized by mixinggraphite with sulfuric acid and potassium permanganate. Further, wateris added thereto, whereby a mixed solution 1 containing the graphiteoxide is formed.

After that, in order to remove the remaining oxidizer, hydrogen peroxideand water may be added to the mixed solution 1. Unreacted potassiumpermanganate is reduced by the hydrogen peroxide and then the reducedpotassium permanganate is reacted with sulfuric acid, whereby manganesesulfate can be formed. Since the manganese sulfate is aqueous, it can beseparated from the graphite oxide insoluble in water.

<Collection of Graphite Oxide>

Next, as shown in Step S122, the graphite oxide is collected from themixed solution 1. The mixed solution 1 is subjected to at least one offiltration, centrifugation, and the like, so that a precipitate 1containing the graphite oxide is collected from the mixed solution 1.Note that the precipitate 1 contains unreacted graphite in some cases.

<Washing of Graphite Oxide>

Next, as shown in Step S123, a metal ion and a sulfate ion are removedfrom the precipitate 1 containing the graphite oxide with an acidsolution. Here, metal ion derived from the oxidizer, which is containedin the graphite oxide, are dissolved in the acid solution, whereby themetal ion and sulfate ion can be removed from the graphite oxide.

Thus, the use of an acid solution for the washing of the graphite oxidecan increase the yields of graphene oxide and graphene oxide salt. Forthis reason, the method for forming graphene oxide in FIG. 2 canincrease the productivity of graphene oxide, further, the productivityof graphene.

Typical examples of the acid solution include hydrochloric acid, dilutesulfuric acid, and nitric acid. Note that the graphite oxide ispreferably washed with a highly-volatile acid typified by hydrochloricacid because the remaining acid solution is easily removed in asubsequent drying step.

As a method for removing a metal ion and a sulfate ion from theprecipitate 1, there are a method in which the precipitate 1 and an acidsolution are mixed and then a mixed solution is subjected to at leastone of filtration, centrifugation, dialysis, and the like; a method inwhich the precipitate 1 is provided over filter paper and then an acidsolution is poured on the precipitate 1; and the like. Here, theprecipitate 1 is provided over filter paper, a metal ion and a sulfateion are removed from the precipitate 1 by washing with the acidsolution, and a precipitate 2 containing the graphite oxide iscollected. Note that the precipitate 2 contains unreacted graphite insome cases.

<Formation of Graphene Oxide>

Next, as shown in Step S124, the precipitate 2 is mixed with water and amixed solution 2 in which the precipitate 2 is dispersed is formed.Then, the graphite oxide contained in the mixed solution 2 is separatedto form graphene oxide. Examples of a method for separating the graphiteoxide to form graphene oxide include application of ultrasonic waves andmechanical stirring. Note that the mixed solution in which the grapheneoxide is dispersed is a mixed solution 3.

The graphene oxide formed through this process contains six-memberedrings each composed of carbon atoms, which are connected in the planardirection, and many-membered rings such as a seven-membered ring, aneight-membered ring, a nine-membered ring, and a ten-membered ring. Notethat the many-membered ring is formed when a carbon bond in part of asix-membered ring composed of carbon atoms is broken and the brokencarbon bond is bonded to a carbon skeleton ring so that the number ofcarbon atoms in the carbon skeleton ring increases. A region surroundedwith carbon atoms in the many-membered ring becomes a gap. An epoxygroup, a carbonyl group such as a carboxyl group, a hydroxyl group, orthe like is bonded to a part of the carbon atoms in the six-memberedring and the many-membered ring. Note that instead of the dispersedgraphene oxide, multilayer graphene oxide may be dispersed.

<Collection of Graphene Oxide>

Next, as shown in Step S125, the mixed solution 3 is subjected to atleast one of filtration, centrifugation, and the like, whereby a mixedsolution containing the graphene oxide and a precipitate 3 containingthe graphite are separated from each other and the mixed solutioncontaining the graphene oxide is collected. Note that the mixed solutioncontaining the graphene oxide is a mixed solution 4. In particular,graphene oxide containing a carbonyl group is ionized and differentgraphene oxides are more likely to be dispersed because hydrogen isionized in a mixed solution having a polarity.

The mixed solution 4 formed through the above step can be used as thedispersion liquid used in Step S111 shown in FIG. 1A.

The mixed solution 4 may contain not a few impurities; thus, it ispreferable to purify the graphene oxide contained in the mixed solution4 formed in Step S125 in order to increase the purity of graphene formedby the method for forming graphene of one embodiment of the presentinvention. Specifically, it is preferable to perform Steps S126 and S127after Step S125. Steps S126 and S127 will be described below.

<Formation of Graphene Oxide Salt>

As shown in Step S126, after a basic solution is mixed into the mixedsolution 4 to form graphene oxide salt, an organic solvent is added, anda mixed solution 5 in which the graphene oxide salt is precipitated as aprecipitate 4 is formed.

As the basic solution, it is preferable to use a mixed solution whichcontains a base that reacts with the graphene oxide in a neutralizationreaction without removing an oxygen atom bonded to a carbon atom of thegrapheme oxide by reducing the graphene oxide. Typical examples of thebasic solution include an aqueous sodium hydroxide solution, an aqueouspotassium hydroxide solution, an aqueous ammonia solution, a methylaminesolution, an ethanolamine solution, a dimethylamine solution, andtrimethylamine solution.

The organic solvent is used to precipitate the graphene oxide salt;thus, acetone, methanol, ethanol, or the like is typically used as theorganic solvent.

<Collection of Graphene Oxide Salt>

Next, as shown in Step S127, the mixed solution 5 is subjected to atleast one of filtration, centrifugation, and the like, whereby thesolvent and the precipitate 4 containing the graphene oxide salt areseparated from each other, and the precipitate 4 containing the grapheneoxide salt is collected.

Next, the precipitate 4 is dried to yield the graphene oxide salt.

When a suspension formed by dispersing the graphene oxide salt formedthrough the above steps in a solvent is used as the dispersion liquid inStep S111 shown in FIG. 1A, graphene formed by the method for forminggraphene of one embodiment of the present invention can have higherpurity.

Note that in a step following Step S123 in FIG. 2, not graphene oxidebut graphite oxide salt may be formed (Step S134), the graphite oxidesalt may be collected (Step S135), and then graphene oxide salt may beformed (see FIG. 3).

Step S134 is as follows. The precipitate 2 obtained in Step S123 ismixed with water, and then a basic solution is mixed into the mixture toform graphite oxide salt. After that, an organic solvent is added to thegraphite oxide salt, and a mixed solution in which the graphite oxidesalt is precipitated is formed. The basic solution can be selected fromthose used in Step S126, and the organic solvent can be selected fromthose used in Step S126.

In Step S135, the mixed solution in which the graphite oxide saltobtained in Step S134 is precipitated is subjected to at least one offiltration, centrifugation, and the like, whereby the organic solventand the precipitate containing the graphite oxide salt are separatedfrom each other, and the precipitate containing the graphite oxide saltis collected.

The other steps in the method for forming graphene oxide salt in FIG. 3are the same as those shown in FIG. 2.

According to this embodiment, graphene which has a higher proportion ofC(sp²)-C(sp²) double bonds and higher conductivity than graphene formedthrough heat treatment can be formed.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 2

In this embodiment, a power storage device of one embodiment of thepresent invention will be described. Specifically, a power storagedevice including an electrode formed by the formation method ofgraphene, which is described in Embodiment 1, will be described. Notethat in this embodiment, description will be given assuming that thepower storage device of one embodiment of the present invention is alithium secondary battery.

First, a positive electrode 311 will be described.

FIG. 4A is a cross-sectional view of a positive electrode 311. In thepositive electrode 311, a positive electrode active material layer 309is formed over a positive electrode current collector 307. The positiveelectrode active material layer 309 includes at least a positiveelectrode active material 321 and graphene 323 (not illustrated) and mayfurther include binder, a conductive additive, and/or the like.

Note that an active material refers to a material that relates toinsertion and extraction of ions serving as carriers (hereinafterreferred to as carrier ions) in a power storage device. Thus, the activematerial and the active material layer are distinguished.

As the positive electrode current collector 307, a material having highconductivity such as platinum, aluminum, copper, titanium, or stainlesssteel can be used. The positive electrode current collector 307 can havea foil shape, a plate shape, a net shape, or the like as appropriate.

As a material of a positive electrode active material 321 contained inthe positive electrode active material layer 309, a lithium compoundsuch as LiFeO₂, LiCoO₂, LiNiO₂, or LiMn₂O₄, or V₂O₅, Cr₂O₅, MnO₂, or thelike can be used.

Alternatively, an olivine-type lithium-containing phosphate (LiMPO₄(general formula) (M is one or more of Fe(II), Mn(II), Co(II), andNi(II))) can be used for the positive electrode active material 321.Typical examples of the general formula LiMPO₄ which can be used as amaterial are lithium compounds such as LiFePO₄, LiNiPO₄, LiCoPO₄,LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄,LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1),LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(d)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1), andLiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1).

Alternatively, a lithium-containing silicate such as Li₂MSiO₄ (generalformula) (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II)) can beused for the positive electrode active material 321. Typical examples ofthe general formula Li₂MSiO₄ which can be used as a material are lithiumcompounds such as Li₂FeSiO₄,Li₂NiSiO₄, Li₂CoSiO₄, Li₂MnSiO₄,Li₂Fe_(k)Ni_(l)SiO₄, Li₂Fe_(k)Co_(t)SiO₄, Li₂Fe_(k)Mn_(l)SiO₄,Li₂Ni_(k)Co_(l)SiO₄, Li₂Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1, and 0<l<1),Li₂Fe_(m)Ni_(n)Co_(q)SiO₄, Li₂Fe_(m)Ni_(n)Mn_(q)SiO₄,Li₂Ni_(m)Co_(n)Mn_(q)SiO₄ (m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi₂Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄(r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1, and0<u<1).

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, thepositive electrode active material 321 may contain a compound obtainedby substituting an alkali metal which is the same kind as a metal of thecarrier ions (e.g., sodium or potassium), an alkaline-earth metal (e.g.,calcium, strontium, or barium), beryllium, or magnesium for lithium inthe lithium compound.

As illustrated in FIG. 4B, which is a plan view of part of the positiveelectrode active material layer 309, the positive electrode activematerial layer 309 includes positive electrode active materials 321which are particles capable of occluding and releasing carrier ions, andgraphenes 323 which cover a plurality of particles of the positiveelectrode active materials 321 and at least partly surround theplurality of particles of the positive electrode active materials 321.Further, in the positive electrode active material layer 309 in the planview, the different graphenes 323 cover surfaces of the plurality ofparticles of the positive electrode active materials 321. Note that thepositive electrode active materials 321 may be exposed in part of thepositive electrode active material layer 309.

The size of the particle of the positive electrode active material 321is preferably 20 nm to 100 nm inclusive. Note that the size of theparticle of the positive electrode active material 321 is preferablysmaller so that the surface area of the positive electrode activematerials 321 is increased and the distance of electrons (and carrierions) transfer is shortened, because electrons (and carrier ions)transfer in the positive electrode active material layer 309.

Sufficient characteristics of a power storage device can be obtainedeven when surfaces of the positive electrode active materials 321 arenot coated with a carbon film; however, it is preferable to use both thegraphene and the positive electrode active material coated with a carbonfilm because current flows between the positive electrode activematerials 321 by hopping conduction.

FIG. 4C is a cross-sectional view of part of the positive electrodeactive material layer 309 in FIG. 4B. FIG. 4C illustrates the positiveelectrode active materials 321 and the graphenes 323 which cover thepositive electrode active materials 321 in the positive electrode activematerial layer 309 in the plan view. The graphenes 323 are observed tohave linear shapes in cross section. One graphene or plural graphenesoverlap with the plurality of particles of the positive electrode activematerials 321, or the plurality of particles of the positive electrodeactive materials 321 are at least partly surrounded with one graphene orplural graphenes. Note that the graphene 323 has a bag-like shape, andthe plurality particles of the positive electrode active materials areat least partly surrounded with the bag-like portion in some cases. Thegraphene partly has openings where the positive electrode activematerials 321 are exposed in some cases.

The desired thickness of the positive electrode active material layer309 is determined in the range of 20 μm to 100 μm. It is preferable toadjust the thickness of the positive electrode active material layer 309as appropriate no that a crack and separation are not caused.

The positive electrode active material layer 309 may contain a knownconductive additive such as acetylene black particles having a volume0.1 to 10 times as large as that of the graphene, or carbon particleshaving a one-dimensional expansion (e.g., carbon nanofibers), and/or aknown binder such as polyvinylidene difluoride (PVDF).

As an example of the positive electrode active material, a materialwhose volume is expanded by occlusion of carrier ions is given. Whensuch a material is used as the positive electrode active material, thepositive electrode active material layer gets vulnerable and is partlycollapsed by charge and discharge, resulting in lower reliability (e.g.,inferior cycle characteristics) of a power storage device. However, thegraphene 323 covering the periphery of the positive electrode activematerials 321 in the positive electrode in the power storage device ofone embodiment of the present invention can prevent the positiveelectrode active materials 321 from being pulverized and can prevent thepositive electrode active material layer 309 from being collapsed, evenwhen the volume of the positive electrode active materials 321 isincreased/decreased due to charge/discharge. That is to say, thegraphene 323 included in the positive electrode in the power storagedevice of one embodiment of the present invention has a function ofmaintaining the bond between the positive electrode active materials 321even when the volume of the positive electrode active materials 321 isincreased/decreased due to charge/discharge. Thus, the use of thepositive electrode 311 allows an improvement in durability of the powerstorage device.

That is to say, binder does not have to be used in forming the positiveelectrode active material layer 309. Therefore, the proportion of thepositive electrode active materials in the positive electrode activematerial layer with certain weight can be increased, leading to anincrease in charge and discharge capacity per unit weight of theelectrode.

The graphene 323 has conductivity and is in contact with a plurality ofparticles of the positive electrode active materials 321; thus, it alsoserves as a conductive additive. For this reason, binder does not haveto be used in forming the positive electrode active material layer 309.Accordingly, the proportion of the positive electrode active materialsin the positive electrode active material layer with certain weight canbe increased, leading to an increase in charge and discharge capacity ofa power storage device per unit weight of the electrode.

Further, the graphene 323 is graphene of one embodiment of the presentinvention. That is, the graphene 323 is obtained by electrochemicalreduction with electric energy and has higher conductivity than grapheneobtained by reduction through heat treatment, as described inEmbodiment 1. A sufficient conductive path (conductive path of carrierions) is formed efficiently in the positive electrode active materiallayer 309, so that the positive electrode active material layer 309 andthe positive electrode 311 have high conductivity. Accordingly, thecapacity of the positive electrode active material 321 in the powerstorage device including the positive electrode 311, which is almostequivalent to the theoretical capacity, can be utilized efficiently;thus, the discharge capacity can be sufficiently high.

Next, a formation method of the positive electrode 311 will bedescribed.

Slurry containing the particulate positive electrode active materials321 and graphene oxide is formed. Specifically, the particulate positiveelectrode active materials 321 and a dispersion liquid containinggraphene oxide are mixed to form the slurry. Note that the dispersionliquid containing graphene oxide can be formed by the method describedin Embodiment 1.

After the positive electrode current collector 307 is coated with theslurry, drying is performed for a certain period of time to remove asolvent from the slurry coating the positive electrode current collector307. For the details, refer to Embodiment 1 as appropriate. Note that inthis case, molding may be performed by applying pressure as needed.

Then, the graphene oxide is electrochemically reduced with electricenergy to the graphene 323 as in the formation method of graphene inEmbodiment 1. Through the above process, the positive electrode activematerial layer 309 can be formed over the positive electrode currentcollector 307, whereby the positive electrode 311 can be formed.

When the positive electrode 311 is formed, the graphene oxide isnegatively charged in a polar solvent because the graphene oxidecontains oxygen. As a result of being negatively charged, the grapheneoxide is dispersed. Accordingly, the positive electrode active materials321 contained in the slurry are not easily aggregated, so that the sizeof the particle of the positive electrode active material 321 can beprevented from increasing in the formation process of the positiveelectrode 311. Thus, it is possible to prevent an increase in internalresistance and the transfer of electrons (and carrier ions) in thepositive electrode active material 321 is easy, leading to highconductivity of the positive electrode active material layer 309 and thepositive electrode 311.

Note that when the positive electrode 311 is formed, the step ofreducing the graphene oxide to form the graphene 323 may be performedafter fabrication of a power storage device including a negativeelectrode, an electrolyte, and a separator. In other words, a potentialat which reduction reaction of the graphene oxide occurs may be suppliedto the positive electrode current collector 307 after fabrication of thepower storage device.

Next, a negative electrode and a formation method thereof will bedescribed.

FIG. 5A is a cross-sectional view of a negative electrode 205. In thenegative electrode 205, a negative electrode active material layer 203is formed over a negative electrode current collector 201. The negativeelectrode active material layer 203 includes at least a negativeelectrode active material 211 and graphene 213 and may further includebinder and/or a conductive additive.

As the negative electrode current collector 201, a material having highconductivity such as copper, stainless steel, iron, or nickel can beused. The negative electrode current collector 201 can have a foilshape, a plate shape, a mesh shape, or the like as appropriate.

The negative electrode active material layer 203 is formed using thenegative electrode active material 211 capable of occluding andreleasing carrier ions. As typical examples of the negative electrodeactive material 211, lithium, aluminum, graphite, silicon, tin, andgermanium are given. Further, a compound containing one or more oflithium, aluminum, graphite, silicon, tin, and germanium is given. Notethat it is possible to omit the negative electrode current collector 201and use the negative electrode active material layer 203 alone for thenegative electrode. The theoretical capacity of germanium, silicon,lithium, and aluminum as the negative electrode active material 211 ishigher than that of graphite as the negative electrode active material211. When the theoretical capacity is high, the amount of negativeelectrode active material can be reduced, so that reductions in cost andsize of a power storage device can be achieved.

FIG. 5B is a plan view of part of the negative electrode active materiallayer 203. The negative electrode active material layer 203 includesnegative electrode active materials 211, which are particles, and thegraphenes 213 which cover a plurality of particles of the negativeelectrode active materials 211 and at least partly surround theplurality of particles of the negative electrode active materials 211.The different graphenes 213 cover surfaces of the plurality of particlesof the negative electrode active materials 211. The negative electrodeactive materials 211 may partly be exposed.

FIG. 5C is a cross-sectional view of part of the negative electrodeactive material layer 203 in FIG. 5B. FIG. 5C illustrates the negativeelectrode active materials 211 and the graphenes 213. The graphenes 213cover a plurality of the negative electrode active materials 211 in thenegative electrode active material layer 203 in the plan view. Thegraphenes 213 are observed to have linear shapes in cross section. Onegraphene or plural graphenes overlap with the plurality of particles ofthe negative electrode active materials 211, or the plurality ofparticles of the negative electrode active materials 211 are at leastpartly surrounded with one graphene or plural graphenes. Note that thegraphene 213 has a bag-like shape, and the plurality particles of thenegative electrode active materials are at least partly surrounded withthe bag-like portion in some cases. The graphene 213 partly has openingswhere the negative electrode active materials 211 are exposed in somecases.

The desired thickness of the negative electrode active material layer203 is determined in the range of 20 μm to 100 μm.

The negative electrode active material layer 203 may contain a knownconductive additive such as acetylene black particles having a volume0.1 to 10 times as large as that of the graphene, or carbon particleshaving a one-dimensional expansion (e.g., carbon nanofibers), and/or aknown binder such as polyvinylidene difluoride.

The negative electrode active material layer 203 may be predoped withlithium in such a manner that a lithium layer is formed on a surface ofthe negative electrode active material layer 203 by a sputtering method.Alternatively, lithium foil is provided on the surface of the negativeelectrode active material layer 203, whereby the negative electrodeactive material layer 203 can be predoped with lithium. Particularly inthe case of forming the graphene 323 on the positive electrode activematerial layer 309 in the positive electrode 311 after fabrication of apower storage device, the negative electrode active material layer 203is preferably predoped with lithium.

As an example of the negative electrode active material 211, a materialwhose volume is expanded by occlusion of carrier ions is given. Whensuch a material is used, the negative electrode active material layergets vulnerable and is partly collapsed by charge and discharge,resulting in lower reliability (e.g., inferior cycle characteristics) ofa power storage device. However, the graphene 213 covering the peripheryof the negative electrode active materials 211 in the negative electrodein the power storage device of one embodiment of the present inventioncan prevent the negative electrode active materials 211 from beingpulverized and can prevent the negative electrode active material layer203 from being collapsed, even when the volume of the negative electrodeactive materials 211 is increased/decreased due to charge/discharge.That is to say, the graphene 213 included in the negative electrode inthe power storage device of one embodiment of the present invention hasa function of maintaining the bond between the negative electrode activematerials 211 even when the volume of the negative electrode activematerials 211 is increased/decreased due to charge/discharge. Thus, theuse of the negative electrode 205 allows an improvement in durability ofthe power storage device.

That is to say, binder does not have to be used in forming the negativeelectrode active material layer 203. Therefore, the proportion of thenegative electrode active materials in the negative electrode activematerial layer with certain weight can be increased, leading to anincrease in discharge capacity per unit weight of the electrode.

The graphene 213 has conductivity and is in contact with a plurality ofparticles of the negative electrode active materials 211; thus, it alsoserves as a conductive additive. Thus, binder does not have to be usedin forming the negative electrode active material layer 203.Accordingly, the proportion of the negative electrode active materialsin the negative electrode active material layer with certain weight(certain volume) can be increased, leading to an increase in charge anddischarge capacity per unit weight (unit volume) of the electrode.

Further, the graphene 213 is graphene of one embodiment of the presentinvention. That is, the graphene 213 is obtained by electrochemicalreduction with electric energy and has higher conductivity than grapheneobtained by reduction through heat treatment, as described inEmbodiment 1. A sufficient conductive path (conductive path of carrierions) is formed efficiently in the negative electrode active materiallayer 203, so that the negative electrode active material layer 203 andthe negative electrode 205 have high conductivity. Accordingly, thecapacity of the negative electrode active material 211 in a powerstorage device including the negative electrode 205, which is almostequivalent to the theoretical capacity, can be utilized as efficiently;thus, the discharge capacity can be sufficiently high.

Note that the graphene 213 also functions as a negative electrode activematerial capable of occluding and releasing carrier ions, leading to anincrease in charge capacity of the negative electrode 205.

Next, a formation method of the negative electrode active material layer203 in FIGS. 5B and 5C will be described.

Slurry containing the particulate negative electrode active materials211 and graphene oxide is formed. Specifically, the particulate negativeelectrode active materials 211 and a dispersion liquid containinggraphene oxide are mixed to form the slurry. The dispersion liquidcontaining graphene oxide can be formed by the method described inEmbodiment 1.

After the negative electrode current collector 201 is coated with theslurry, drying is performed in vacuum for a certain period of time toremove a solvent from the slurry coating the negative electrode currentcollector 201. For the details, refer to Embodiment 1 as appropriate.Note that in this case, molding may be performed by applying pressure asneeded.

Then, the graphene oxide is electrochemically reduced with electricenergy to the graphene 213 as in the formation method of graphene inEmbodiment 1. Through the above process, the negative electrode activematerial layer 203 can be formed over the negative electrode currentcollector 201, whereby the negative electrode 205 can be formed.

In the case where graphene in the positive electrode 311 and thenegative electrode 205 is formed by the method described in Embodiment 1in fabricating a power storage device including the positive electrode311 and the negative electrode 205, it is preferable to form graphene ineither the positive electrode 311 or the negative electrode 205 inadvance before fabrication of the power storage device. This is becausewhen the power storage device is fabricated with graphene oxide providedin the positive electrode 311 and the negative electrode 205, potentialcannot be efficiently supplied to the positive electrode 311 and thenegative electrode 205, so that the graphene oxide is reducedinsufficiently or it takes a long time to sufficiently reduce thegraphene oxide.

When the negative electrode 205 is formed, the graphene oxide isnegatively charged in a polar solvent because it contains oxygen. As aresult of being negatively charged, the graphene oxide is dispersed.Accordingly, the negative electrode active materials 211 contained inthe slurry are not easily aggregated, so that the size of the particleof the negative electrode active material 211 can be prevented fromincreasing in the formation process of the negative electrode 205. Thus,it is possible to prevent an increase in internal resistance and thetransfer of electrons (and carrier ions) in the negative electrodeactive material 211 is easy, leading to high conductivity of thenegative electrode active material layer 203 and the negative electrode205.

Next, the structure of a negative electrode in FIG. 5D will bedescribed.

FIG. 5D is a cross-sectional view of the negative electrode where thenegative electrode active material layer 203 is formed over the negativeelectrode current collector 201. The negative electrode active materiallayer 203 includes a negative electrode active material 221 having anuneven surface and graphene 223 covering a surface of the negativeelectrode active material 221.

The uneven negative electrode active material 221 includes a commonportion 221 a and a projected portion 221 b extending from the commonportion 221 a. The projected portion 221 b can have a columnar shapesuch as a cylinder shape or a prism shape, or a needle shape such as acone shape or a pyramid shape as appropriate. The top portion of theprojected portion may be curved. The negative electrode active material221 is formed using a negative electrode active material capable ofoccluding and releasing carrier ions (typically, lithium ions) similarlyto the negative electrode active material 211. Note that the commonportion 221 a and the projected portion 221 b may be formed using eitherthe same material or different materials.

In the case of silicon which is an example of a negative electrodeactive material, the volume is approximately quadrupled due to occlusionof ions serving as carriers; therefore, the negative electrode activematerial gets vulnerable and is partly collapsed by charge anddischarge, resulting in lower reliability (e.g., inferior cyclecharacteristics) of a power storage device. However, when silicon isused as the negative electrode active material 221 in the negativeelectrode illustrated in FIG. 5D, the graphene 223 covering theperiphery of the negative electrode active material 221 can prevent thenegative electrode active material 221 from being pulverized and canprevent the negative electrode active material layer 203 from beingcollapsed, even when the volume of the negative electrode activematerial 221 is increased/decreased due to charge/discharge.

When a surface of a negative electrode active material layer is incontact with an electrolyte contained in a power storage device, theelectrolyte and the negative electrode active material react with eachother, so that a film is formed on a surface of a negative electrode.The film is called a solid electrolyte interface (SEI) and considerednecessary to relieve the reaction of the negative electrode and theelectrolyte for stabilization. However, when the thickness of the filmis increased, carrier ions are less likely to be occluded in thenegative electrode, leading to problems such as a reduction inconductivity of carrier ions between the electrode and the electrolyteand a waste of the electrolyte.

The graphene 213 coating the surface of the negative electrode activematerial layer 203 can prevent an increase in thickness of the film, sothat a decrease in charge and discharge capacity can be prevented.

Next, a formation method of the negative electrode active material layer203 in FIG. 5D will be described.

The uneven negative electrode active material 221 is provided over thenegative electrode current collector 201 by a printing method, anink-jet method, a CVD method, or the like. Alternatively, a negativeelectrode active material having a film shape is formed by a coatingmethod, a sputtering method, an evaporation method, or the like, andthen is selectively removed, so that the uneven negative electrodeactive material 221 is provided over the negative electrode currentcollector 201. Still alternatively, a surface of foil or a plate whichis formed of lithium, aluminum, graphite, or silicon is partly removedto form the negative electrode current collector 201 and the negativeelectrode active material 221 that have an uneven shape. Furtheralternatively, a net formed of lithium, aluminum, graphite, or siliconmay be used for the negative electrode active material and the negativeelectrode current collector.

Then, the uneven negative electrode active material 221 is coated with adispersion liquid containing graphene oxide. As a method for applyingthe dispersion liquid containing graphene oxide, the method described inEmbodiment 1 may be employed as appropriate.

Subsequently, a solvent in the dispersion liquid containing grapheneoxide is removed as described in Embodiment 1. After that, electricenergy may be used to electrochemically reduce the graphene oxide toform the graphene 213, as described in Embodiment 1.

When the graphene is thus formed with the use of the dispersion liquidcontaining graphene oxide, the surface of the uneven negative electrodeactive material 221 can be coated with the graphene 213 with an eventhickness.

In the case where graphene in the positive electrode 311 and thenegative electrode illustrated in FIG. 5D is formed by the methoddescribed in Embodiment 1 in fabricating a power storage deviceincluding the positive electrode 311 and the negative electrode, it ispreferable to form the graphene in either the positive electrode 311 orthe negative electrode in advance before fabrication of the powerstorage device. This is because when the power storage device isfabricated with graphene oxide provided in the positive electrode 311and the negative electrode, potential cannot be efficiently supplied tothe positive electrode 311 and the negative electrode, so that thegraphene oxide is reduced insufficiently or it takes a long time tosufficiently reduce the graphene oxide.

Note that the uneven negative electrode active material 221 (hereinafterreferred to as silicon whiskers) formed of silicon can be provided overthe negative electrode current collector 201 by an LPCVD method usingsilane, silane chloride, silane fluoride, or the like as a source gas.

The silicon whiskers may be amorphous. When amorphous silicon whiskersare used for the negative electrode active material layer 203, thevolume is less likely to be changed due to occlusion and release ofcarrier ions (e.g., stress caused by expansion in volume is relieved).For this reason, the silicon whiskers and the negative electrode activematerial layer 203 can be prevented from being pulverized and collapsed,respectively, due to repeated cycles of charge and discharge;accordingly, a power storage device can have further improved cyclecharacteristics.

Alternatively, the silicon whisker may be crystalline. In this case, thecrystalline structure having excellent conductivity and carrier ionmobility is in contact with the current collector in a wide range ofarea. Therefore, it is possible to further improve the conductivity ofthe entire negative electrode, which enables charge and discharge to beperformed at much higher speed; accordingly, a power storage devicewhose charge and discharge capacity is improved can be fabricated.

Still alternatively, the silicon whisker may include a core, which is acrystalline region, and an outer shell covering the core, which is anamorphous region.

The amorphous outer shell has a characteristic that the volume is lesslikely to be changed due to occlusion and release of carrier ions (e.g.,stress caused by expansion in volume is relieved). In addition, thecrystalline core, which has excellent conductivity and ion mobility, hasa characteristic that the rate of occluding ions and the rate ofreleasing ions are high per unit mass. Thus, when the silicon whiskerhaving the core and the outer shell is used for the negative electrodeactive material layer, charging and discharging can be performed at highspeed; accordingly, a power storage device whose charge and dischargecapacity and cycle characteristics are improved can be fabricated.

Next, how to fabricate a power storage device of one embodiment of thepresent invention will be described. FIG. 6 is a cross-sectional view ofa lithium secondary battery 400, and the cross-sectional structurethereof will be described below.

A lithium secondary battery 400 includes a negative electrode 411including a negative electrode current collector 407 and a negativeelectrode active material layer 409, a positive electrode 405 includinga positive electrode current collector 401 and a positive electrodeactive material layer 403, and a separator 413 provided between thenegative electrode 411 and the positive electrode 405. Note that theseparator 413 is impregnated with an electrolyte 415. The negativeelectrode current collector 407 is connected to an external terminal 419and the positive electrode current collector 401 is connected to anexternal terminal 417. An end portion of the external terminal 419 isembedded in a gasket 421. That is to say, the external terminals 417 and419 are insulated from each other by the gasket 421.

As the negative electrode current collector 407 and the negativeelectrode active material layer 409, the negative electrode currentcollector 201 and the negative electrode active material layer 203,which are described above, can be used as appropriate.

As the positive electrode current collector 401 and the positiveelectrode active material layer 403, the positive electrode currentcollector 307 and the positive electrode active material layer 309,which are described above, can be used as appropriate.

As the separator 413, an insulating porous material is used. Typicalexamples of the separator 413 include paper; nonwoven fabric; a glassfiber; ceramics; and synthetic fiber containing nylon (polyamide),vinylon (polyvinyl alcohol based fiber), polyester, acrylic, polyolefin,or polyurethane. Note that a material which is not dissolved in theelectrolyte 415 needs to be selected.

When a positive electrode provided with a spacer over the positiveelectrode active material layer is used as the positive electrode 405,the separator 413 does not necessarily have to be provided.

As a solute of the electrolyte 415, a material which contains carrierions is used. Typical examples of the solute of the electrolyte includelithium salts such as LiClO₄, LiAsF₆, LiBF₄, LiPF₅, and Li(C₂F₅SO₂)₂N.

Note that when carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions,instead of lithium in the above lithium salts, an alkali metal (e.g.,sodium or potassium), an alkaline-earth metal (e.g., calcium, strontium,or barium), beryllium, or magnesium may be used for a solute of theelectrolyte 415.

As a solvent of the electrolyte 415, a material in which lithium ionscan transfer is used. As the solvent of the electrolyte 415, an aproticorganic solvent is preferably used. Typical examples of aprotic organicsolvents include ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, γ-butyrolactone, acetonitrile,dimethoxyethane, tetrahydrofuran, and the like, and one or more of thesematerials can be used. When a gelled high-molecular material is used asthe solvent of the electrolyte 415, safety against liquid leakage andthe like is improved. Further, the lithium secondary battery 400 can bethinner and more lightweight. Typical examples of gelled high-molecularmaterials include a silicon gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide, polypropylene oxide, a fluorine-based polymer, andthe like. Alternatively, the use of one or more of ionic liquids (roomtemperature molten salts) which are less likely to burn and volatilizeas a solvent of the electrolyte 415 can prevent a power storage devicefrom exploding or catching fire even when the power storage deviceinternally shorts out or the internal temperature increases due toovercharging or the like.

As the electrolyte 415, a solid electrolyte such as Li₃PO₄ can be used.Other examples of the solid electrolyte include Li_(x)PO_(y)N_(z) (x, y,and z are positive real numbers) which is formed by mixing Li₃PO₄ withnitrogen; Li₂S—SiS₂; Li₂S—P₂S₅; and Li₂S—B₂S₃. Any of the above solidelectrolytes which is doped with LiI or the like may be used. Note thatin the case of using such a solid electrolyte as the electrolyte 415,the separator 413 is unnecessary.

For the external terminals 417 and 419, a metal material such as astainless steel plate or an aluminum plate can be used as appropriate.

Note that in this embodiment, a coin-type lithium-ion secondary batteryis given as the lithium secondary battery 400; however, any of lithiumsecondary batteries with various shapes, such as a sealing-type lithiumsecondary battery, a cylindrical lithium secondary battery, and asquare-type lithium secondary battery, can be used. Further, a structurein which a plurality of positive electrodes, a plurality of negativeelectrodes, and a plurality of separators are stacked or rolled may beemployed.

A lithium secondary battery has a small memory effect, a high energydensity, a large capacity, and a high output voltage, which enablesreduction in size and weight. Further, the lithium-ion secondary batterydoes not easily deteriorate due to repeated charge and discharge and canbe used for a long time, so that cost can be reduced.

The formation methods of a positive electrode and a negative electrode,which are described in Embodiment 1 and this embodiment, are employed asappropriate to form the positive electrode 405 and the negativeelectrode 411.

Next, the positive electrode 405, the separator 413, and the negativeelectrode 411, are impregnated with the electrolyte 415. Then, thepositive electrode 405, the separator 413, the gasket 421, the negativeelectrode 411, and the external terminal 419 are stacked in this orderover the external terminal 417, and the external terminal 417 and theexternal terminal 419 are crimped to each other with a “coin cellcrimper”. Thus, the coin-type lithium secondary battery can bemanufactured.

Note that a spacer and a washer may be provided between the externalterminal 417 and the positive electrode 405 or between the externalterminal 419 and the negative electrode 411 so that the connectionbetween the external terminal 417 and the positive electrode 405 orbetween the external terminal 419 and the negative electrode 411 isenhanced.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 3

The power storage device of one embodiment of the present invention canbe used for power supplies of a variety of electric appliances which canbe operated with electric power.

Specific examples of electric appliances each utilizing the powerstorage device of one embodiment of the present invention are asfollows: display devices, lighting devices, desktop personal computersand laptop personal computers, image reproduction devices whichreproduce still images and moving images stored in recording media suchas digital versatile discs (DVDs), mobile phones, portable gamemachines, portable information terminals, e-book readers, video cameras,digital still cameras, high-frequency heating appliances such asmicrowave ovens, electric rice cookers, electric washing machines,air-conditioning systems such as air conditioners, electricrefrigerators, electric freezers, electric refrigerator-freezers,freezers for preserving DNA, and dialyzers. In addition, moving objectsdriven by electric motors using electric power from power storagedevices are also included in the category of electric appliances.Examples of the moving objects include electric vehicles, hybridvehicles each including both an internal-combustion engine and anelectric motor, and motorized bicycles including motor-assistedbicycles.

In the electric appliances, the power storage device of one embodimentof the present invention can be used as a power storage device forsupplying enough electric power for almost the whole electric powerconsumption (referred to as a main power supply). Alternatively, in theelectric appliances, the power storage device of one embodiment of thepresent invention can be used as a power storage device which can supplyelectric power to the electric appliances when the supply of electricpower from the main power supply or a commercial power supply is stopped(such a power storage device is referred to as an uninterruptible powersupply). Still alternatively, in the electric appliances, the powerstorage device of one embodiment of the present invention can be used asa power storage device for supplying electric power to the electricappliances at the same time as the power supply from the main powersupply or a commercial power supply (such a power storage device isreferred to as an auxiliary power supply).

FIG. 7 illustrates specific structures of the electric appliances. InFIG. 7, a display device 5000 is an example of an electric applianceincluding a power storage device 5004. Specifically, the display device5000 corresponds to a display device for TV broadcast reception andincludes a housing 5001, a display portion 5002, speaker portions 5003,and the power storage device 5004. The power storage device 5004 isprovided in the housing 5001. The power storage device of one embodimentof the present invention is used as the power storage device 5004. Thedisplay device 5000 can receive electric power from a commercial powersupply. Alternatively, the display device 5000 can use electric powerstored in the power storage device 5004. Thus, the display device 5000can be operated with the use of the power storage device 5004 as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 5002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 7, an installation lighting device 5100 is an example of anelectric appliance including a power storage device 5103. Specifically,the lighting device 5100 includes a housing 5101, a light source 5102,and the power storage device 5103. The power storage device of oneembodiment of the present invention is used as the power storage device5103. Although FIG. 7 illustrates the case where the power storagedevice 5103 is provided in a ceiling 5104 on which the housing 5101 andthe light source 5102 are installed, the power storage device 5103 maybe provided in the housing 5101. The lighting device 5100 can receiveelectric power from a commercial power supply. Alternatively, thelighting device 5100 can use electric power stored in the power storagedevice 5103. Thus, the lighting device 5100 can be operated with the useof the power storage device 5103 as an uninterruptible power supply evenwhen electric power cannot be supplied from a commercial power supplydue to power failure or the like.

Note that although the installation lighting device 5100 provided in theceiling 5104 is illustrated in FIG. 7 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 5105, afloor 5106, a window 5107, or the like other than the ceiling 5104.Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 5102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 7, an air conditioner including an indoor unit 5200 and anoutdoor unit 5204 is an example of an electric appliance including apower storage device 5203. Specifically, the indoor unit 5200 includes ahousing 5201, an air outlet 5202, and the power storage device 5203. Thepower storage device of one embodiment of the present invention is usedas the power storage device 5203. Although FIG. 7 illustrates the casewhere the power storage device 5203 is provided in the indoor unit 5200,the power storage device 5203 may be provided in the outdoor unit 5204.Alternatively, the power storage devices 5203 may be provided in boththe indoor unit 5200 and the outdoor unit 5204. The air conditioner canreceive electric power from a commercial power supply. Alternatively,the air conditioner can use electric power stored in the power storagedevice 5203. Particularly in the case where the power storage devices5203 are provided in both the indoor unit 5200 and the outdoor unit5204, the air conditioner can be operated with the use of the powerstorage device 5203 of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 7 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 7, an electric refrigerator-freezer 5300 is an example of anelectric appliance including a power storage device 5304 of oneembodiment of the present invention. Specifically, the electricrefrigerator-freezer 5300 includes a housing 5301, a door for arefrigerator 5302, a door for a freezer 5303, and the power storagedevice 5304. The power storage device of one embodiment of the presentinvention is used as the power storage device 5304. The power storagedevice 5304 is provided in the housing 5301 in FIG. 7. The electricrefrigerator-freezer 5300 can receive electric power from a commercialpower supply. Alternatively, the electric refrigerator-freezer 5300 canuse electric power stored in the power storage device 5304. Thus, theelectric refrigerator-freezer 5300 can be operated with the use of thepower storage device 5304 as an uninterruptible power supply even whenelectric power cannot be supplied from a commercial power supply due topower failure or the like.

Note that among the electric appliances described above, ahigh-frequency heating apparatus such as a microwave oven and anelectric appliance such as an electric rice cooker require high power ina short time. The tripping of a breaker of a commercial power supply inuse of an electric appliance can be prevented by using the power storagedevice of one embodiment of the present invention as an auxiliary powersupply for supplying electric power which cannot be supplied enough by acommercial power supply.

In addition, in a time period when electric appliances are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the power storage device, whereby the usage rate ofelectric power can be reduced in a time period when the electricappliances are used. For example, in the case of the electricrefrigerator-freezer 5300, electric power can be stored in the powerstorage device 5304 in night time when the temperature is low and thedoor for a refrigerator 5302 and the door for a freezer 5303 are notoften opened or closed. On the other hand, in daytime when thetemperature is high and the door for a refrigerator 5302 and the doorfor a freezer 5303 are frequently opened and closed, the power storagedevice 5304 is used as an auxiliary power supply; thus, the usage rateof electric power in daytime can be reduced.

Next, a personal digital assistant including a power storage device ofone embodiment of the present invention will be described with referenceto FIGS. 8A to 8C.

FIGS. 8A and 8B illustrate a tablet terminal that can be folded. FIG. 8Aillustrates the tablet terminal in the state of being unfolded. Thetablet terminal includes a housing 9630, a display portion 9631 a, adisplay portion 9631 b, a display-mode switching button 9034, a powerbutton 9035, a power-saving-mode switching button 9036, a fastener 9033,and an operation button 9038.

A touch panel area 9632 a can be provided in part of the display portion9631 a, in which area, data can be input by touching displayed operationkeys 9637. Note that half of the display portion 9631 a has only adisplay function and the other half has a touch panel function. However,the structure of the display portion 9631 a is not limited to this, andall the area of the display portion 9631 a may have a touch panelfunction. For example, a keyboard can be displayed on the whole displayportion 9631 a to be used as a touch panel, and the display portion 9631b can be used as a display screen.

A touch panel area 9632 b can be provided in part of the display portion9631 b like in the display portion 9631 a. When a keyboard displayswitching button 9639 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

The touch panel area 9632 a and the touch panel area 9632 b can becontrolled by touch input at the same time.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 9036allows optimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for determining inclination,such as a gyroscope or an acceleration sensor, may be incorporated inthe tablet terminal.

Although the display area of the display portion 9631 a is the same asthat of the display portion 9631 b in FIG. 8A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 9631 a may be different from that of the displayportion 9631 b, and further, the display quality of the display portion9631 a may be different from that of the display portion 9631 b. Forexample, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

FIG. 8B illustrates the tablet terminal in the state of being closed.The tablet terminal includes the housing 9630, a solar cell 9633, acharge/discharge control circuit 9634, a battery 9635, and a DC-DCconverter 9636. FIG. 8B illustrates an example where thecharge/discharge control circuit 9634 includes the battery 9635 and theDC-DC converter 9636. A power storage device of one embodiment of thepresent invention is used as the battery 9635.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not in use. Thus, the display portions 9631a and 9631 b can be protected, which permits the tablet terminal to havehigh durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 8A and 8B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 9633, which is attached on a surface of the tabletterminal, can supply electric power to a touch panel, a display portion,an image signal processor, and the like. Note that a structure in whichthe solar cell 9633 is provided on one or two surfaces of the housing9630 is preferable to charge the battery 9635 efficiently. The use of apower storage device of one embodiment of the present invention as thebattery 9635 has advantages such as a reduction in size.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 8B will be described with reference to a blockdiagram of FIG. 8C. FIG. 8C illustrates the solar cell 9633, the battery9635, the DC-DC converter 9636, a converter 9638, switches SW1 to SW3,and the display portion 9631. The battery 9635, the DC-DC converter9636, the converter 9638, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 8B.

First, an example of operation in the case where electric power isgenerated by the solar cell 9633 using external light will be described.The voltage of electric power generated by the solar cell is raised orlowered by the DC-DC converter 9636 so that the electric power has avoltage for charging the battery 9635. When the display portion 9631 isoperated with the electric power from the solar cell 9633, the switchSW1 is turned on and the voltage of the electric power is raised orlowered by the converter 9638 to a voltage needed for operating thedisplay portion 9631. In addition, when display on the display portion9631 is not performed, the switch SW1 is turned off and the switch SW2is turned on so that the battery 9635 may be charged.

Although the solar cell 9633 is described as an example of a powergeneration means, there is no particular limitation on the powergeneration means, and the battery 9635 may be charged with any of theother means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 9635 maybe charged with a non-contact power transmission module capable ofperforming charging by transmitting and receiving electric powerwirelessly (without contact), or any of the other charge means used incombination.

It is needless to say that an embodiment of the present invention is notlimited to the electric equipment illustrated in FIGS. 8A to 8C as longas the electric equipment is equipped with the power storage devicedescribed in the above embodiment.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Example 1

In this example, a lithium secondary battery (referred to as a lithiumsecondary battery 1) was fabricated according to one embodiment of thepresent invention and measured by cyclic voltammetry (CV).

First, the structure and the fabrication method of the lithium secondarybattery 1 will be described.

The lithium secondary battery 1 was a coin lithium secondary battery. Asa working electrode of the lithium secondary battery, an electrode inwhich an active material layer including LiFePO₄ and graphene oxide wasprovided over a current collector made of aluminum was used. As acounter electrode and a reference electrode, lithium metals were used.As a separator, a polypropylene sheet was used. As an electrolyte, amixed solution in which 1M of LiPF₆ (ethylene carbonate solvent) anddiethyl carbonate were mixed at a ratio (volume ratio) of 1:1 was used.

Here, a method for forming the working electrode will be described.

<Synthesis Method of LiFePO₄>

Lithium carbonate (Li₂CO₃), iron oxalate (Fe₂CO₄.2H₂O), and ammoniumdihydrogen phosphate (NH₄H₂PO₄), which were materials, were weighed sothat the molar ratio of Li₂CO₃:Fe₂CO₄.2H₂O:NH₄H₂PO₄ was 1:2:2. Then, thematerials were ground and mixed with a wet ball mill (the ball diameterwas 3 mm and acetone was used as a solvent) at 300 rpm for two hours.

Next, the ground and mixed materials were subjected to pre-baking at350° C. in a nitrogen atmosphere for ten hours and then ground and mixedwith a wet ball mill (the ball diameter was 3 mm and acetone was used asa solvent) at 300 rpm for two hours again. After that, baking wasperformed at 600° C. in a nitrogen atmosphere for ten hours to yieldLiFePO₄.

<Synthesis Method of Graphene Oxide>

To form the mixed solution A, 2 g of graphite and 92 ml of concentratedsulfuric acid were mixed. Then, 12 g of potassium permanganate was addedto the mixed solution A while they were stirred in an ice bath, so thatthe mixed solution B was formed. After the ice bath was removed andstirring was performed at room temperature for two hours, the resultingsolution was left at 35° C. for 30 minutes so that the graphite wasoxidized. Consequently, the mixed solution C containing graphite oxidewas formed.

Next, 184 ml of water was added to the mixed solution C while they werestirred in an ice bath, so that a mixed solution D was formed. After themixed solution D was stirred in an oil bath at about 98° C. for 15minutes so that reaction was caused, 580 ml of water and 36 ml ofhydrogen peroxide solution (with a concentration of 30 wt %) were addedto the mixed solution D while they were stirred, in order to reduceunreacted potassium permanganate. Consequently, a mixed solution Econtaining soluble manganese sulfate and the graphite oxide was formed.

After the mixed solution E was subjected to suction filtration using amembrane filter with a hole diameter of 0.45 μm to give the precipitateA, the precipitate A and 3 wt % of hydrochloric acid were mixed, so thata mixed solution F in which a manganese ion, a potassium ion, and asulfate ion were dissolved was formed. After that, the mixed solution Fwas subjected to suction filtration to give the precipitate B containingthe graphite oxide.

After the precipitate B was mixed with 500 ml of water to form a mixedsolution G, ultrasonic waves with a frequency of 40 kHz were applied tothe mixed solution G for one hour to separate carbon layers in thegraphite oxide from each other, so that graphene oxide was formed.

Next, centrifugation was carried out at 4000 rpm for about 30 minutes,and a supernatant fluid containing the graphene oxide was collected. Thesupernatant fluid is a mixed solution H.

Next, ammonia water was added to the mixed solution H so that the mixedsolution has a pH of 11, whereby a mixed solution 1 was formed. Afterthat, 2500 ml of acetone was added to the mixed solution 1 and they weremixed to form a mixed solution J. At this time, the graphene oxidecontained in the mixed solution H reacted with ammonia contained in theammonia water to form graphene oxide salt (specifically, ammonium saltof graphene oxide) as a precipitate in the mixed solution J.

The mixed solution J was filtrated, and the precipitate in the mixedsolution J was dried at room temperature in vacuum to collect thegraphene oxide salt.

<Formation Method of Active Material Layer>

The working electrode in which an active material layer was providedover the current collector was formed in such a manner that 97 wt %LiFePO₄ and 3 wt % graphene oxide salt were mixed with NMP(N-methylpyrrolidone) having a weight about twice as large as the totalweight of the LiFePO₄ and the graphene oxide salt to form a paste, thepaste was applied to the current collector made of aluminum, ventilationdrying was performed at 120° C. for 15 minutes, and then the currentcollector was heated to 100° C. and drying was performed for one hour invacuum.

Next, the process of fabricating the lithium secondary battery 1 will bedescribed. At the beginning, in a first battery can, the workingelectrode was provided so as to be immersed in the electrolyte, theseparator was provided over the working electrode so as to be immersedin the electrolyte, and a gasket was provided over the separator. Then,a lithium metal was provided over the separator and the gasket, and aspacer and a spring washer were provided over the lithium electrode.After a second battery can was provided over the spring washer, thefirst battery can was crimped. In this manner, the lithium secondarybattery 1 was fabricated.

Next, CV measurement of the lithium secondary battery 1 was performed.The sweep rate was 1 mV/s. In the first step under the condition thatthe sweep potential was 3 V to 4 V, a cycle in which a suppliedpotential was swept from 3 V to 4 V and then swept from 4 V to 3 V wasrepeated four times. In the second step under the condition that thesweep potential was 1.5 V to 3 V, a cycle in which a supplied potentialwas swept from 3 V to 1.5 V and then swept from 1.5 V to 3 V wasrepeated four times. In the third step under the condition that thesweep potential was 3 V to 4 V, a cycle in which a supplied potentialwas swept from 3 V to 4 V and then swept from 4 V to 3 V was repeatedfour times. FIG. 9 shows current-potential curves in this case.

In FIG. 9, the horizontal axis represents potential of the workingelectrode (vs. Li/Li⁺), and the vertical axis represents currentgenerated by reduction-oxidation. Note that negative current valuesindicate reduction current, and positive current values indicateoxidation current.

A current having a peak surrounded by a broken line 501_R is a reductioncurrent in the first step, and a current having a peak surrounded by abroken line 501_O is an oxidation current in the first step. A currenthaving a peak surrounded by a broken line 502_R is a reduction currentin first potential sweeping in the second step, and a current shown by abroken line 502 is a reduction current in second to fourth potentialsweeping in the second step and an oxidation current in first to fourthpotential sweeping in the second step. A current having a peaksurrounded by a broken line 503_R is a reduction current in the thirdstep, and a current having a peak surrounded by a broken line 503_O isan oxidation current in the third step.

The graph shows that the current value of the lithium secondary battery1 was increased due to the potential sweeping from 1.5 V to 3 V, in thefirst to third steps. In other words, the graph shows that theresistance of the active material layer was decreased due to reductiontreatment where a potential for promoting reduction reaction of theactive material layer is supplied, i.e., electrochemical reductiontreatment, and the current value was increased in the third step. Giventhe fact that the redox potential of LiFePO₄ included in the activematerial layer is approximately 3.4 V, it can be said that a reductioncurrent around 2 V was generated when the graphene oxide was reduced,which suggests that the reduction potential of the graphene oxide wasapproximately 2 V.

FIG. 10 is an enlarged graph showing the current-potential curves in thesecond step in FIG. 9.

In FIG. 10, a curve 511_R represents a reduction current in the firstpotential sweeping, and a curve 511_O represents an oxidation current inthe first potential sweeping. Further, a curve 512_R represents areduction current in the second to fourth potential sweeping, and acurve 512_O represents an oxidation current in the second to fourthpotential sweeping.

As shown in FIG. 10, the reduction current in the first potentialsweeping has a peak at around 2V. In contrast, the reduction current inthe second and later potential sweeping does not have a peak at around 2V. The oxidation current in the first to fourth potential sweeping doesnot have a significant change.

The measurement results reveal that the reduction reaction of theworking electrode occurred due to the potential sweeping at 2 V, whichwas the reduction potential, whereas the reduction reaction did notoccur in the second and later potential sweeping.

Here, in order to examine the reduction reaction caused at around 2 V, acomparative battery cell in which an active layer of a working electrodeincluded only graphene oxide was fabricated and CV measurement thereofwas performed.

First, the structure and the fabrication method of the comparativebattery cell will be described.

The comparative battery cell was a coin battery. The comparative batterycell had the same structure as the lithium secondary battery 1 exceptthat the active material layer of the working electrode, which includedonly graphene oxide, was provided over a current collector made ofaluminum.

The graphene oxide was formed through steps similar to those of thegraphene oxide used for the active material layer of the workingelectrode in the lithium secondary battery 1.

The working electrode in which the active material layer was providedover the current collector made of aluminum was formed in such a mannerthat 50 mg of graphene oxide was mixed with 4.5 g of water to form apaste, the paste was applied to the current collector, and drying wasperformed at 40° C. in vacuum.

The process of fabricating the comparative battery cell was similar tothat of the lithium secondary battery 1.

Next, CV measurement of the comparative battery cell was performed. Thesweep rate was 0.1 mV/s. Under the condition that the sweep potentialwas 1.5 V to 3 V, a cycle in which a supplied potential was swept from 3V to 1.5 V and then swept from 1.5 V to 3 V was repeated three times.FIG. 11 shows current-potential curves in this case.

In FIG. 11, the horizontal axis represents potential of the workingelectrode (vs. Li/Li⁺), and the vertical axis represents currentgenerated by reduction-oxidation. A curve 531_R represents a reductioncurrent in the first potential sweeping, and a curve 531_O represents anoxidation current in the first potential sweeping. A curve 532_Rrepresents a reduction current in the second potential sweeping, and acurve 532_O represents an oxidation current in the second potentialsweeping. A curve 533_R represents a reduction current in the thirdpotential sweeping, and a curve 533_O represents an oxidation current inthe third potential sweeping.

As shown in FIG. 11, the reduction current in the first potentialsweeping has a peak at around 2 V. This result suggests that thereduction potential of the graphene oxide was approximately 2 V. Incontrast, the reduction current in the second and later potentialsweeping does not have a peak at around 2 V. Although the oxidationcurrent in the second and third potential sweeping is larger than thatin the first potential sweeping, the oxidation current in the second andthird potential sweeping does not have a significant change.

FIGS. 12 and 13 show X-ray photoelectron spectroscopy (XPS) analysisresults of the surface elemental composition of carbon, oxygen, andanother element, and the states of the atomic bonds before and afterelectrochemical reduction treatment of the working electrode ofcomparative battery cells.

A sample 1 was formed by providing the mixed solution H containinggraphene oxide, which is described in the formation steps of the workingelectrode of the lithium secondary battery 1, over a substrate made ofaluminum and performing heating at 40° C. in vacuum for one hour. Asample 2 was formed by immersing the sample 1 in the electrolytecontained in the lithium secondary battery 1 for one day, performingwashing with diethyl carbonate, and then performing drying at roomtemperature in vacuum for three hours. Note that the sample 1 and thesample 2 are samples before electrochemical reduction treatment. Asample 3 was formed in such a manner that a working electrode obtainedby disassembling the comparative battery cell on which CV measurementwas performed once was washed with diethyl carbonate, and drying wasperformed at room temperature in vacuum for three hours.

On the other hand, a sample obtained using a method for forming graphenenot by electrochemical reduction of graphene oxide but by thermalreduction of graphene oxide and a sample formed using graphite were usedas comparative examples.

A sample formed in such a manner that powdered graphene oxide obtainedby drying the mixed solution H containing graphene oxide, which isdescribed in the formation process of the lithium secondary battery 1,was provided over indium foil was used as a comparative example 1. Asample formed in such a manner that graphene obtained by heating thecomparative example 1 at 300° C. in vacuum for ten hours to reduce thegraphene oxide was provided over indium foil was used as a comparativeexample 2. A sample formed by providing powdered graphite over indiumfoil was used as a comparative example 3.

FIG. 12 shows XPS analysis results of the surface elemental compositionin the samples 1 to 3 and the comparative examples 1 to 3.

FIG. 12 shows that the proportion of oxygen in the sample 3 was lowerthan that of each of the sample 1 and the sample 2, that the proportionof carbon in the sample 3 was higher than that of each of the sample 1and the sample 2, and that the proportion of oxygen in the sample 3obtained by electrochemical reduction was 14.8 at. %. FIG. 12 also showsthat the proportion of oxygen in the comparative example 2 was lowerthan that of the comparative example 1 and the proportion of oxygen inthe comparative example 2 obtained by thermal reduction was 13.4 at. %.The above results indicate that the graphene oxide was reduced byelectrochemical reduction. The above results also indicate that thegraphene oxide was reduced by thermal reduction.

FIG. 13 shows XPS analysis results of the states of the atomic bonds ofnear-surfaces of the samples 1 to 3 and the comparative examples 1 to 3.

FIG. 13 is a graph showing the evaluated proportions of sp² bonds of Cdenoted as C═C, sp³ bonds of C such as C—C and C—H, C—O bonds, C═Obonds, CO₂ bonds (O═C—O bond), and CF₂ bonds.

The graph shows that the proportion of sp² bonds of C denoted as C═C inthe sample 3 was higher than that of each of the sample 1 and the sample2 and the proportions of sp³ bonds of C such as C—C and C—H, C—O bonds,C═O bonds, and CO₂ bonds were lower than those of each of the sample 1and the sample 2. These results reveal that electrochemical reductiontreatment caused the reaction of sp³ bonds, C—O bonds, C═O bonds, andCO₂ bonds, so that sp² bonds were formed. The proportion of sp² bonds inthe sample 3 was 67.2%.

The graph also shows that the proportion of sp² bonds in the comparativeexample 2 was higher than that of the comparative example 1, as in thesample 3, but was lower than that of the sample 3. The proportion of sp²bonds in the comparative example 2 was 44.1%. That is to say, theseresults suggest that when electrochemical reduction treatment isperformed, the proportion of sp² bonds becomes 50% to 70% inclusive.

Thus, FIGS. 11 to 13 indicate that the graphene oxide was reduced due tothe sweeping of the reduction potential at around 2 V, so that graphenewith many sp² bonds was formed. Further, FIGS. 10 and 12 show that theresistance of the active material layer was reduced due to the sweepingof the reduction potential at around 2 V, leading to an increase incurrent value of the lithium secondary battery. The analysis results inFIGS. 11 to 13 suggest that the resistance was reduced because thegraphene oxide with low conductivity was reduced by electrochemicalreduction to form graphene with high conductivity.

Example 2

In this example, the reduction potential of graphene oxide which wasmeasured with a measurement system without electrode resistancecomponents will be described.

It can be said that the resistance of the entire electrode includinggraphene oxide which was formed by the method described in Example 1 washigh.

In this example, graphene oxide was sparsely attached to an electrode,and the reduction potential of the graphene oxide was measured with themeasurement system, from which resistance components generated when thegraphene oxide was stacked were removed.

Specifically, glassy carbon serving as a working electrode and platinumserving as a counter electrode were immersed in a graphene oxidedispersion liquid in which graphene oxide was dispersed in water as asolvent at 0.0027 g/L, and a voltage of 10 V was applied to the workingelectrode and the counter electrode for 30 seconds. After that, theglassy carbon to which graphene oxide was attached was dried in vacuum.Here, the glassy carbon to which graphene oxide was attached is agraphene oxide electrode A. Note that the graphene oxide used in thisexample was formed as in Example 1.

Thus, when electrophoresis in the graphene oxide dispersion liquid wasperformed while conditions were controlled, so that graphene oxide wasable to be sparsely attached to glassy carbon serving as the workingelectrode.

Then, the graphene oxide electrode A, platinum, and lithium were used asa working electrode, a counter electrode, and a reference electrode,respectively, and CV measurement was performed. Note that in the CVmeasurement, a solution in which 1M LiPF₆ was dissolved in a mixedsolution in which EC and DEC were mixed at a ratio of 1:1 was used as anelectrolyte.

For the sweep rates in the CV measurement, the following threeconditions were used: 10 mV/s (condition 1), 50 mV/s (condition 2), and250 mV/s (condition 3). The range of sweep potential was the same in allthe conditions 1 to 3. Potential sweeping was performed from a lowerpotential to a higher potential and from the higher potential to thelower potential, in the range of 1.8 V to 3.0 V from the immersionpotential, three times.

FIGS. 15A and 15B and FIG. 16A show CV measurement results under theconditions 1 to 3. FIG. 15A shows results under the condition 1. FIG.15B shows results under the condition 2. FIG. 16A shows results underthe condition 3. FIG. 16B shows CV measurement results of a comparativeexample formed using only glassy carbon as a working electrode. Thecondition for the CV measurement of the comparative example was the sameas the condition 2 except that the potential sweeping was performedtwice. Note that in FIGS. 15A and 15B and FIGS. 16A and 16B, thehorizontal axis represents potential of the working electrode (vs.Li/Li⁺), and the vertical axis represents current generated byreduction-oxidation.

FIG. 16B shows that in the comparative example in which graphene oxidewas not attached to the working electrode, the redox reaction did notoccur in the range of 1.8 V to 3.0 V.

On the other hand, in the results under the conditions 1 to 3, in thecase of the graphene oxide electrode A, to which graphene oxide wasattached, only peaks in the first potential sweeping are observed at 2.3V and 2.6 V as irreversible reduction reactions. No peak in the secondand third potential sweeping is observed as in the case of thecomparative example (see FIGS. 15A and 15B and FIG. 16A).

Further, the results under the conditions 1 to 3 indicate that althoughthere were differences in current flowing to the measurement systemdepending on the potential sweep rate, the positions of the peaks didnot depend on the potential sweep rate and were approximately 2.3 V andapproximately 2.6 V under all the conditions.

Thus, the peaks observed at 2.3 V and 2.6 V presumably correspond to thereduction reaction of the graphene oxide.

According to one embodiment of the present invention, graphene can beformed probably due to the supply of a potential at which the reductionreaction of graphene oxide occurs.

REFERENCE NUMERALS

S111: step, S112: step, S121: step, S122: step, S123: step, S124: step,S125: step, S126: step, S127: step, 113: container, 114: electrolyte,115: conductive layer, 116: counter electrode, 201: negative electrodecurrent collector, 203: negative electrode active material layer, 205:negative electrode, 211: negative electrode active material, 213:graphene, 221: negative electrode active material, 221 a: commonportion, 221 b: projected portion, 223: graphene, 307: positiveelectrode current collector, 309: positive electrode active materiallayer, 311: positive electrode, 321: positive electrode active material,323: graphene, 400: lithium secondary battery, 401: positive electrodecurrent collector, 403: positive electrode active material layer, 405:positive electrode, 407: negative electrode current collector, 409:negative electrode active material layer, 411: negative electrode, 413:separator, 415: electrolyte, 417: external terminal, 419: externalterminal, 421: gasket, 501_O: broken line, 501_R: broken line, 502:broken line, 502_R: broken line, 503_O: broken line, 503_R: broken line,511_O: curve, 511_R: curve, 512_O: curve, 512_R: curve, 531_O: curve,531_R: curve, 532_O: curve, 532_R: curve, 533_O: curve, 533_R: curve,5000: display device, 5001: housing, 5002: display portion, 5003:speaker portion, 5004: power storage device, 5100: lighting device,5101: housing, 5102: light source, 5103: power storage device, 5104:ceiling, 5105: wall, 5106: floor, 5107: window, 5200: indoor unit, 5201:housing, 5202: air outlet, 5203: power storage device, 5204: outdoorunit, 5300: electric refrigerator-freezer, 5301: housing, 5302: door forrefrigerator, 5303: door for freezer, 5304: power storage device, 9630:housing, 9631: display portion, 9631 a: display portion, 9631 b: displayportion, 9632 a: touch panel area, 9632 b: touch panel area, 9033:fastener, 9034: display-mode switching button, 9035: power button, 9036:power-saving-mode switching button, 9038: operation button, 9639:keyboard display switching button, 9633: solar cell, 9634: charge anddischarge control circuit, 9635: battery, 9636: DC-DC converter, 9637:operation key, and 9638: converter

This application is based on Japanese Patent Application serial no.2011-217897 filed with the Japan Patent Office on Sep. 30, 2011, theentire contents of which are hereby incorporated by reference.

What is claimed is:
 1. An electrode for a lithium ion batterycomprising: a current collector; and an active material layer over thecurrent collector, the active material layer comprising: a particle ofan active material comprising an alkali metal; and a graphene comprisinghydrogen atoms, carbon atoms and oxygen atoms, wherein the graphene is aone-atom-thick sheet or a stack of 2 to 100 layers of the sheet, whereinthe graphene has a two-dimensional structure and has a sheet-like shape,wherein a proportion of the carbon atoms measured by X-ray photoelectronspectroscopy is higher than or equal to 80% and lower than 90%, whereina proportion of the oxygen atoms measured by X-ray photoelectronspectroscopy is higher than or equal to 10% and lower than 20%, whereina sum of the proportion of the carbon atoms and the proportion of theoxygen atoms is lower than 100%, wherein, in bonds of the carbon atoms,a proportion of sp²-bonded carbon atoms among the carbon atoms measuredby X-ray photoelectron spectroscopy is higher than or equal to 50% andlower than or equal to 80%, and wherein the graphene and the particle ofthe active material are randomly dispersed in the active material layer.2. A power storage device comprising: an electrolyte comprising alithium salt; and an electrode according to claim
 1. 3. An electrodecomprising: a current collector; and an active material layer over thecurrent collector, the active material layer comprising: a particle ofan active material comprising an alkali metal; and a materialcomprising: hydrogen atoms; carbon atoms whose proportion measured byX-ray photoelectron spectroscopy is higher than or equal to 80% andlower than or equal to 90%; and oxygen atoms whose proportion measuredby X-ray photoelectron spectroscopy is higher than or equal to 10% andlower than or equal to 20%, wherein the material is a one-atom-thicksheet or a stack of 2 to 100 layers of the sheets, wherein the materialhas a two-dimensional structure and has a sheet-like shape, and whereina proportion of sp²-bonded carbon atoms of the carbon atoms is higherthan or equal to 50% and lower than or equal to 80%, and wherein thematerial and the particle of the active material are randomly dispersedin the active material layer.
 4. A power storage device comprising: theelectrode according to claim 3; an electrolyte; and a separator.
 5. Thepower storage device according to claim 4, wherein the electrolyte is anaprotic organic solvent.
 6. A power storage device comprising: anelectrolyte comprising lithium salt; and an electrode comprising: acurrent collector; and an active material layer over the currentcollector, the active material layer comprising: a particle of an activematerial; and a graphene comprising hydrogen atoms, carbon atoms andoxygen atoms, wherein the graphene is a one-atom-thick sheet or a stackof 2 to 100 layers of the sheet, wherein the graphene has atwo-dimensional structure and has a sheet-like shape, wherein aproportion of the carbon atoms measured by X-ray photoelectronspectroscopy is higher than or equal to 80% and lower than 90%, whereina proportion of the oxygen atoms measured by X-ray photoelectronspectroscopy is higher than or equal to 10% and lower than 20%, whereina sum of the proportion of the carbon atoms and the proportion of theoxygen atoms is lower than 100%, wherein, in bonds of the carbon atoms,a proportion of sp²-bonded carbon atoms among the carbon atoms measuredby X-ray photoelectron spectroscopy is higher than or equal to 50% andlower than or equal to 80%, and wherein the graphene and the particle ofthe active material are randomly dispersed in the active material layer.7. The electrode according to claim 1, wherein the active materialcomprises an olivine-type lithium-containing phosphate.
 8. The electrodeaccording to claim 3, wherein the active material comprises anolivine-type lithium-containing phosphate.
 9. The electrode according toclaim 6, wherein the active material comprises an olivine-typelithium-containing phosphate.
 10. The electrode according to claim 1,wherein the graphene covers the particle of the active material.
 11. Theelectrode according to claim 3, wherein the material covers the particleof the active material.
 12. The power storage device according to claim6, wherein the graphene covers the active material.
 13. The electrodeaccording to claim 1, wherein a size of the particle of the activematerial is 20 nm or more and 100 nm or less.
 14. The electrodeaccording to claim 1, wherein a thickness of the electrode is in a rangeof 20 μm to 100 μm.